Cookies per site

The cookie count is a proxy. It tells you how many vendors run on the page. Most cookies exist for tracking and each tracker sets its own. The count grows with every tag the marketing stack adds. Cookies are also sent with every request to their domain, which adds header weight. That is rarely the real cost.

The real cost is what the cookies stand for. The vendors behind them run script on your main thread and delay every interaction (INP). They also inject banners, widgets and ads after the page looks settled (CLS). A rising cookie count means it is time to audit the tag manager, not to shrink cookies.

How does this affect the Core Web Vitals?

Cookie size correlates with the LCP. Where the cookie size is low, 85% of sites pass the LCP. Where it is high, 63% do. Cookie size has little causal relationship with the LCP. That makes a high cookie size more likely a result or an indication of a slow, heavily built site than a cause of a slow one. More cookies almost always means more trackers, more third parties and more script, and those do slow pages down.